1. Field of the Invention
This invention relates to the propagation of electromagnetic waves and, more particularly, to a angular filter comprising an array of elements which interact with the electromagnetic waves as a function of the angle of incidence of a wave upon a surface of the filter.
2. Description of the Prior Art
An angular filter, also referred to as a spatial filter, is a device which passes or attenuates an electromagnetic wave depending on the angle of incidence of the wave relative to a surface of the filter. Typically, such filters are designed to pass a wave propagating at normal incidence (broadside) and to provide attenuation or rejection that increase with increasing angle of incidence away from broadside. The filter may be employed in combination with a directive antenna of electromagnetic radiation, in which application the filter serves to reduce sidelobes in the radiation pattern of the antenna.
Several types of angular filters have been described in the literature including, by way of example, multilayered dielectric filters (R. J. Mailloux, "Synthesis of Spatial Filters with Chebyshev Characteristics", IEEE Trans. Antennas and Progagation, pp. 174-181; March 1976), perforated metal sheet filters (E. L. Rope, G. Tricoles, "An Angle Filter Containing Three Periodically Perforated Metallic Layers", IEEE AP-S Int. Symp. Digest, pp. 818-820; 1979) and multilayered metal-grid filters (R. J. Mailloux, "Studies of Metallic Grid Spatial Filters", IEEE Int. Symp. Digest, p. 551, 1977; P.R. Franchi, R. J. Mailloux, "Theoretical and Experimental Study of Metal Grid Angular Filters for Sidelobe Suppression", IEEE Trans. Antennas and Propagation, pp, 445-450, May 1983; P. W. Hannan and J. R. Pedersen, "Investigation of Metal Grid Angular Filters", Proc. 1980 Antenna Applications Symposium, Allerton Park, Ill. September 1980; and J. F. Pedersen, P. W. Hannan, "A Metal Grid 5.times.5 Foot Angular Filter", IEEE AP-S Symp. Digest, pp. 471-474, 1982).
Various forms of construction have been utilized in the fabrication of the angular filters resulting in a variety of benefits and limitations. By way of example, metal-grid angular filters are practical and can offer improved performance, such as a reduction in wide-angle sidelobes, when combined with an antenna. However, the metal-grid filters are limited in the useful frequency bandwidth due to the dependency of the filter characteristics on frequency. Also, such filters have an inherent resonant nature necessitating tight dimensional tolerances in their construction. An insufficiency in the tolerances may result in variations of transmission phase across the filter aperture for angles of incidence within the filter angular passband. Such phase variations can create unwanted sidelobes in the radiation pattern produced by the combination of the antenna with the filter.
A further limitation found in filters having the metal grid construction is the rejection of electromagnetic power by reflection rather than by absorption. Such reflected power can return to the antenna, associated with the filter, and then reflect back to the filter. Such multiple reflection yields unwanted sidelobes within the angular passband of the filter. Thus, it is seen that the present forms of construction introduce limitations which detract from the benefits which would otherwise be provided by the angular filters.
One solution to these problems is proposed by my invention for an Axial Conductance Angular Filter described in U.S. Pat. No. 4,604,629 issued Aug. 5, 1986, incorporated herein by reference. That invention is directed to angular filtering for E-plane incidence whereas this invention is directed to angular filtering for H-plane incidence.